Cellulose acetate semipermeable membrane and process for producing the cellulose acetate semipermeable membrane

ABSTRACT

To provide a cellulose acetate semipermeable membrane having a high filtration rate and high mechanical strength and which is hardly degraded by microorganisms, the present invention provides a cellulose acetate semipermeable membrane containing a cellulose acetate having an α-cellulose content of not less than 99% by weight, a 6 wt/vol % viscosity of 20 to 220 mPa·s at 25±1° C. and an acetylation degree of 58 to 62%. The cellulose acetate semipermeable membrane is suitable particularly as a hollow fiber membrane.

FIELD OF THE INVENTION

The present invention relates to a cellulose acetate semipermeablemembrane used for separation and concentration treatment in variousfields besides water treatments such as drinking water treatment, sewagetreatment and waste water treatment, and a process for producing thecellulose acetate semipermeable membrane.

PRIOR ART

Heretofore, a cellulose acetate membrane has been used as a material ofseparation membrane for various kinds of water treatment and for medicaluse in blood dialysis etc., for the reason of hydrophilicity and alittle decrease in filtration rate, and widely used at presentparticularly in the form of a reverse osmosis membrane. For demand inpractical use, the reverse osmosis membrane made of cellulose acetate isoften constructed such that a separation active layer in the membrane ismade very dense with a membrane pore diameter of 0.001 μm or less, butthere arise various problems attributable to the membrane structure.

For example, JP-B 58-24164 discloses a reverse osmosis membrane made ofcellulose acetate having a dense layer. However, since this reverseosmosis membrane has a dense layer, the operating pressure should bekept at high pressure as high as 1 MPa or more to increase thefiltration rate, which however leads to problems including not only anincrease in energy cost but also a decrease in water permeation rate andmechanical breakage of the membrane due to the compaction anddensification of the membrane during filtrating operation. Forapplication of this membrane t o a blood dialysis membrane, the membranethickness has been rendered thin to raise the plasma separation rate,but this results in the general problem of easy breakage at lowpressure.

The reverse osmosis membrane made of cellulose acetate has anotherproblem that pinholes are generated due to the presence of micro-voids.JP-B 60-43442 discloses a method for suppressing the generation ofpinholes and for improving membrane strength. However, this prior artdoes not solve the problem of a decrease in filtration rate duringpractical use, because the membrane has a substantially uniformstructure containing no void layer.

JP-A 6-343842 and JP-A 8-108053 disclose a cellulose acetate hollowfiber separation membrane having a three-dimensional network-like partand a void part.

Another requirement of such a membrane material for various kinds ofwater treatment is that it is hardly degraded by microorganisms in orderto suppress a decrease in its filtration ability and to increase thelongevity of the membrane thus preventing an increase in operating cost.

The object of the present invention is to provide a cellulose acetatesemipermeable membrane which has a high filtration rate at a lowpressure, has high mechanical strength and is hardly degraded bymicroorganisms, and a process for producing the cellulose acetatesemipermeable membrane.

Further, JP-A10-305220discloses a hollow fiber membrane fabricated bydischarging a membrane-forming solution through a double pipe typespinning orifice while discharging an inside coagulating solution from acentral pipe of the spinning orifice.

DISCLOSURE OF THE INVENTION

As a result of their eager study on the structure of cellulose acetateserving as the starting material for production of semipermeablemembranes, the present inventor have found that 3 elements relating tothe structure described above, which are specified by connecting themwith one another, can act synergistically to achieve the objectdescribed above, thus completing the present invention.

That is, the present invention provides a cellulose acetatesemipermeable comprising a cellulose acetate produced from cellulosehaving an α-cellulose content of not less than 99% by weight, and havinga 6 wt/vol % viscosity at 25±1° C. of 20 to 220 mPa·s and an acetylationdegree of 58 to 62%. Also, it provides use of the cellulose acetate as asemipermeable membrane or a hollow fiber membrane.

Preferably, the semipermeable membrane is a hollow fiber membrane.

Further, the present invention provides a cellulose acetate hollow fibermembrane, wherein the thickness of the hollow fiber membrane is 100 to400 μm, the cross-section of the hollow fiber membrane is composed of athree-dimensional network-like part and a void part, the void part ispositioned inside 10 μm or more from both internal and external surfacesof the membrane, the area occupied by the void part is in the range of 5to 60% of the total cross-sectional area of the membrane, a dense layerhaving a surface average pore size of 0.001 to 0.05 μm exists on each ofthe internal and external surfaces of the hollow fiber, and a crack-likemuscular pattern (slit structure) is observed on the internal surface ofthe hollow fiber with an electron microscope at a magnification of ×20,000.

Preferably, the cellulose acetate is produced from cellulose having anα-cellulose content of not less than 99% by weight. Preferably, the 6%viscosity thereof is 50 to 200 mPa·s, and the acetylation degree thereofis 60.5 to 61.5%.

Further preferably, the pure water permeation rate is not less than 500l/(m²·h), and the tensile strength at break is not less than 4 MPa.

Further, the present invention provides a process for producing acellulose acetate semipermeable membrane, wherein a solution of theabove-described cellulose acetate dissolved in a water-soluble, organicpolar solvent, for example, dimethyl sulfoxide, N-methyl-2-pyrrolidoneor dimethylacetamide, is used to produce the membrane.

Preferably, the solution is formed into a hollow fiber membrane byapplying a wet or dry wet spinning process using a double pipe typespinning orifice.

Preferably, at least one selected from compounds containing metallicelements of the I to III groups of the periodic table, ethylene glycoland polyethylene glycol are dissolved with cellulose acetate.

Further, the present invention provides a process for producing acellulose acetate hollow fiber membrane wherein the thickness of themembrane is 100 to 400 μm, the cross-section of the hollow fibermembrane is composed of a three-dimensional network-like part and a voidpart, the void part is positioned inside 10 μm or more from bothinternal and external surfaces of the membrane, the area occupied by thevoid part is in the range of 5 to 60% of the total cross-sectional areaof the membrane, a dense layer having a surface average pore diameter of0.001 to 0.05 μm exists on each of the internal and external surfaces ofthe follow fiber, and a crack-like muscular pattern (slit structure) isobserved on the internal surface of the hollow fiber with an electronmicroscope at a magnification of ×20,000, comprising dissolving acellulose acetate in a water-soluble, organic polar solvent, and thendischarging the resulting membrane-forming solution from a double pipenozzle while discharging an inside coagulating solution from the innerpipe of the double pipe to coagulate the membrane-forming solution in acoagulation bath.

Preferably, the linear velocity of the inside coagulating solutiondischarged is 8 times or more as high as the linear velocity of themembrane-forming solution discharged.

Preferably, the membrane-forming solution contains a compound containingmetallic elements of the I to III groups of the periodic table in anamount of 0.2 to 1.5% by weight therein.

In addition, the present invention provides use, as water treatment orseparation and concentration treatment, of a cellulose acetate hollowfiber membrane in which the thickness of the hollow fiber membrane is100 to 400 μm, a cross-section of the hollow fiber membrane consists ofa three-dimensional network-like part and a void part, the void part ispositioned 10 μm or more inside from both internal and external surfacesof the membrane, the area occupied by the void part is in the range of 5to 60% of the total cross-sectional area of the membrane, each of theinternal and external surfaces of the follow fiber has a dense layerhaving a surface average pore size of 0.001 to 0.05 μm, and a crack-likemuscular pattern (slit structure) is observed on the internal surface byobservation under an electron microscope at a magnification of ×20,000.

Further additionally, the present invention provides a method ofconducting water treatment or separation and concentration treatmentusing a cellulose acetate hollow fiber membrane in which the thicknessof the hollow fiber membrane is 100 to 400 μm, a cross-section of thehollow fiber membrane consists of a three-dimensional network-like partand a void part, the void part is positioned 10 μm or more inside fromboth internal and external surfaces of the membrane, the area occupiedby the void part is in the range of 5 to 60% of the totalcross-sectional area of the membrane, each of the internal and externalsurfaces of the hollow fiber has a dense layer having a surface averagepore size of 0.001 to 0.05 μm, and a crack-like muscular pattern (slitstructure) is observed on the internal surface by observation under anelectron microscope at a magnification of ×20,000.

The cellulose acetate semipermeable membrane of the present inventionexhibits a high filtration rate and high mechanical strength and ishardly degraded by microorganisms because the cellulose acetate as thestarting material satisfies the prescribed 3 elements i.e. α-cellulosecontent, 6% viscosity at 25±1° C. and acetylation degree. Further, it isalso excellent in that the procedure of forming the membrane is easy.

The cellulose acetate hollow fiber membrane of the present invention ishighly reliable, since it is excellent in water permeability, has highmechanical strength and shows a suppressed generation of membranedefects such as pinholes.

Further, the membrane is preferable in the case of the followingconditions:

Size of the void part: 10 to 200 μm.

Size of the slit: 0.05 to 1.0 μm length,

0.005 to 0.2 μm width,

slit length/slit width=5 to 50,

at least one slit per μm² is observed under an SEM at a magnification of×20, 000.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, with the explanation of an example of the process forproducing a cellulose acetate semipermeable membrane according to thepresent invention, the cellulose acetate semipermeable membrane isillustrated.

Cellulose acetate as the raw material of the cellulose acetatesemipermeable membrane has a specific structure satisfying theprescribed 3 elements as described below, but the cellulose itself asthe starting material is not particularly limited, and plant cellulosesuch as industrial pulps, linters etc., bacterial cellulose andregenerated cellulose such as rayon may be used. Among these, cottonlinters are preferable.

The cellulose acetate is the one having an α-cellulose content of notless than 99% by weight, preferably not less than 99.5% by weight. Ifthe content of α-cellulose is not less than 99% by weight, a gel contentnot dissolved in the membrane-forming solution is reduced so that thegeneration of pinholes can be suppressed and the strength of themembrane can also be increased.

The 6% based wt/vol, herein after, viscosity at 25±1° C. of thecellulose acetate is 20 to 220 mPa·s, preferably 50 to 180 mPa·s. Whenthe 6% viscosity is within the range defined above, the procedure offorming the membrane is facilitated so that the membrane can be producedeven when the temperature of the membrane-forming solution is kept atrelatively low temperature (100° C. or less).

Further, the cellulose acetate is the one having acetylation degree of58 to 62%, preferably 60 to 62%, particularly preferably 60.5 to 61.5.When the acetylation degree is within the range described above, theresulting membrane is hardly degraded by microorganisms, thus prolongingduration of the membrane and improving the spinning properties thereof.The acetylation degree is determined by the measurement methodprescribed in Examples.

The cellulose acetate used in the present invention is preferably theone wherein the number of insolubles with a particle diameter of 3 to100 μm per mg cellulose acetate is preferably 10 or less, particularlypreferably 5 or less. When the number of insolubles is 10 or less, it ispossible to prevent occurrence of problems such as breakage of themembrane during membrane manufacturing or generation of pinholes bypreventing formation of the membrane structure via phase conversion.Accordingly, in pre-treatment prior to membrane-forming, insolubles inthe cellulose acetate are preferably removed by filtration under apressure through a solvent-resistant filter having a pore diameter of 10μm or less, preferably 0.5 to 5 μm, such as a sintered metal filter, afilter paper, a filter cloth, a PTFE membrane filter etc.

In the present invention, the above-mentioned cellulose acetate isdissolved in dimethyl sulfoxide, N-methyl-2-pyrrolidone ordimethylacetamide to prepare a membrane-forming solution.

As the solvent, the above-mentioned 3 organic solvents maybe used aloneor in combination thereof as necessary. Along with these solvents,1,4-dioxane, N,N-dimethylformamide, 2-pyrrolidone and/or γ-butyrolactonemay be used in combination.

The amount of the solvent used is such that the concentration ofcellulose acetate in the membrane-forming solution is made preferably 10to 30% by weight, particularly preferably 15 to 23% by weight.

Along with the cellulose acetate, a compound containing metallicelements of the I to III groups of the periodic table may be dissolvedtherein. The compound includes at least one selected from acetates,halides such as chloride etc., nitrates, thiocyanates and hydrates ofalkali metals such as Li, Na and K or alkaline earth metals such as Mgand Ca. Among these, lithium chloride, magnesium chloride, lithiumacetate and magnesium acetate having high solubility are preferable.Further, a non-solvent such as ethylene glycol and polyethylene glycolmay be added.

Preferably, the amount of the above-mentioned compound added is 0.05 to5% by weight and the amount of the non-solvent added is 1 to 30% byweight to the total weight of the membrane-forming solution, in order toimprove both the water permeability and strength of the membrane, and tofacilitate the procedure of forming the membrane by preventing anincrease in viscosity.

The process of forming the membrane varies depending on the type of thedesired membrane, and the process of forming the membrane according tothe type of membrane such as flat membrane, spiral, tube and hollowfiber may be applied. For example, the spinning process using a doublepipe type spinning orifice as described below can be applied to theforming of the hollow fiber membrane. The spinning process may be eithera wet process or a dry wet process.

In the case of the wet process, the membrane-forming solution isdischarged from an outer pipe of a double pipe type spinning orifice,while an inside coagulating solution is discharged from an inner pipe tocoagulate the solution in a coagulation bath. Both the temperature ofthe inside coagulating solution and the temperature of the coagulationbath are preferably in the range of 30 to 80° C., to give the hollowfiber membrane having a dense layer of suitable thickness.

In the case of the dry wet process, the distance of a drying partbetween the discharge part of the spinning orifice and the coagulationbath is preferably 0.1 to 50 cm, particularly preferably 0.3 to 30 cm,and preferably the solution is introduced into the coagulation bath,after passing the air for 0.2 second or more at the distance mentionedabove. The temperature of the coagulation bath is the same temperaturerange as in the wet process.

The solvent used in the inside coagulating solution or in thecoagulation bath is the one which does not dissolve cellulose acetateand is compatible with the solvent used in forming the membrane, and asexamples, water, ethylene glycol, polyethylene glycol etc. may beproposed. The compound used in preparing the membrane-forming solutionmay also be added to the inside coagulating solution or the coagulationbath.

When the cellulose acetate semipermeable membrane of the presentinvention is a hollow fiber membrane, the hollow fiber membranepreferably has the following structure:

The hollow fiber membrane preferably has a dense layer on the internalor external surface of the membrane and has both a three-dimensionalnetwork-like porous part having porosity and a void part in the insideof the membrane.

The dense layer is present in a depth of substantially up to {fraction(1/100)} of the membrane thickness from the internal or external surfaceof the membrane, and the surface average pore diameter is preferably inthe range of 0.001 to 0.05 μm, particularly preferably 0.005 to 0.03 μm,and the numerical range corresponds to molecular weight cut-off of10,000 to 500,000. For example, the decrease in filtration rate causedby penetration of suspending particles in a treated fluid into theinside of the membrane can be prevented by regulating the surfaceaverage pore size within the range defined above.

The three-dimensional network-like porous part is formed in theremainder of the density layer, and the average pore diameter of thevoid is smaller than the average pore diameter of the dense layer, andis substantially in the range of 0.05 to 1 μm. When the average porediameter is in this range, high mechanical strength and ductility can beconferred on the hollow fiber membrane.

The void part coexists with the three-dimensional network-like porouspart and consists of round or elliptical voids (porosity) which arelarger, substantially 10 to 200 μm in size, than voids in thethree-dimensional porous part and having little filtration resistanceagainst a permeating fluid. The ratio of the area occupied by the voidpart to the cross-sectional area of the membrane is preferably 5 to 60%,particularly preferably 20 to 50%. When the ratio of the area occupiedby the void part is within the range defined above, the filtration ratecan be increased, and further mechanical strength such as tensilestrength and burst pressure can be raised.

The membrane thickness is preferably 50 to 500 μm, more preferably 100to 400 μm. When the membrane thickness is within the above range, thefiltration rate and mechanical strength can be raised.

By constructing the hollow fiber membrane as the above structure, thefiltration rate can be raised without deteriorating the mechanicalstrength of the membrane, as compared with a hollow fiber membranehaving a gradient-type porous layer having a continuously increasingpore diameter from the surface of the membrane with the minimum porediameter to the inside of the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron microphotograph at a magnification of ×50 of across-section of the cellulose acetate hollow fiber membrane in Example4 of the present invention.

FIG. 2 is an electron microphotograph at a magnification of ×20,000 ofthe internal surface of the cellulose acetate hollow fiber membrane inExample 4 of the present invention.

FIG. 3 is an electron microphotograph at a magnification of ×50 of across-section of the cellulose acetate hollow fiber membrane inComparative Example 6.

FIG. 4 is an electron microphotograph at a magnification of ×20,000 ofthe internal surface of the cellulose acetate hollow fiber membrane inComparative Example 6.

FIG. 5 is an electron microphotograph at a magnification of ×20,000 ofthe internal surface of the cellulose acetate hollow fiber membrane inExample 5 of the present invention.

FIG. 6 is an electron microphotograph at a magnification of ×20,000 ofthe internal surface of the cellulose acetate hollow fiber membrane inComparative Example 7.

EXAMPLES

Hereinafter, the present invention is described more in detail byreference to Examples, which however are not intended to limit thepresent invention. The measurement of cellulose acetate and celluloseacetate semipermeable membranes given below was conducted in thefollowing method.

(1) 6% viscosity

61.67 g of a mixed solvent (methylene chloride:methanol=91:9 (ratio byweight)) was placed in an Erlenmeyer flask, 3.00 g of a sample dried at105±5° C. for 2 hours was introduced thereinto, and then the flask wassealed. Thereafter, it was shaken for about 1.5 hours in a shaking bathand further shaken for about 1 hour in a rotary shaker, whereby thesample was completely dissolved. Then, the temperature of the resulting6 wt/vol % solution was adjusted to 25±1° C. in a thermostat, and thetime of downward flowing required for the downward flowing solution topass through two marked lines was determined with the Ostwaldviscometer. The viscosity was determined from the following equation:

 6% viscosity (mPa·s)=time of downward flowing (sec)/viscometercoefficient.

The viscometer coefficient was determined from the following equationafter the time (sec) of downward flowing of a standard solution forviscometer calibration was measured in the same manner as above.

Viscometer coefficient=[absolute viscosity of the standard

solution (mPa·s)×density of the solution (1.235

g/cm³)/[density of the standard solution (g/cm³)×time of

downward flowing of the standard solution (sec)]

(2) Acetylation degree

1.9 g of a ground sample was placed in a weighing bottle, dried at105±5° C. for 2 hours, and then cooled as it was for about 40 minutes ina desiccator. It was accurately weighed and transferred to an Erlenmeyerflask, and the emptied weighing bottle was accurately weighted again,and the difference between these weights was assumed to be the weight ofthe sample. After about 70 ml acetone was added to the Erlenmeyer flaskand stirred for about 30 minutes, about 30 ml dimethyl sulfoxide wasadded thereto and stirred for about 10 minutes, whereby the sample wascompletely dissolved. Thereafter, about 50 ml acetone was added theretoand stirred for about 5 minutes. Then, 30 ml of 1 N NaOH was addedthereto under slow stirring, and the mixture was stirred about 2 hours.Thereafter, about 100 ml hot water of 80 to 90° C. was added thereto andstirred for about 15 minutes. A phenolphthalein solution was added as anindicator, and the solution was titrated with 1N H₂SO₄ until its palered color disappeared. The titration amount was expressed as A ml, whilea blank test was conducted and the titration amount thereof wasexpressed as B ml, and the acetylation degree was determined by thefollowing equation. “F” in the equation is the factor of 1N H₂SO₄.

Acetylation degree (%)=[(B−A)×F×6.005]/sample weight  (g)

(3) Pure water permeation rate

The inside of a hollow fiber membrane having an effective length of 50cm was pressurized with pure water at 25° C. at a water pressure of 100kPa to determine the amount of filtered pure water (internal surfacearea standard).

(4) Tensile strength at break

Tensile strength at break, as determined by moving a cross-head at arate of 10 mm/min against a hollow fiber membrane with an effectivelength of 5 cm, was converted to that of per cm² cross-section of thehollow fiber membrane.

(5) Microbial degradability

Cellulose acetate semipermeable membranes were immersed for 100 days inwater from the Ibo river downstream. The semipermeable membranes afterthe immersion whose tensile strength at break after the immersionexceeded 80% of the strength before the immersion were regarded as beingnot microbially degradable, while those coming to have the strength ofnot more than 80% were regarded as being microbially degradable.

Example 1

After stirring and dissolving 20% by weight cellulose acetate shown inTable 1 and 80% by weight dimethyl sulfoxide at 90° C., the resultingsolution was filtered using a filter paper (Toyo Roshi No. 63) to give amembrane-forming solution. The membrane-forming solution was dischargedat a pressure of 0.5 MPa and at a discharge temperature of 90° C. fromoutside of a double pipe type spinning orifice, while water of 70° C. asan inside coagulating solution was discharged from an inner pipe. Afterpassing the air for 0.5 second, it was coagulated in a water bath at 70°C. , spun at a wind-up rate of 12 m/min and then the solvent wasthoroughly removed in a washing bath. The resulting hollow fibermembrane had an inner diameter of 0.8 mm and an outer diameter of 1.3mm. The results of each measurement are shown in Table 1.

Comparative Examples 1 to 3

Hollow fiber membranes with the same dimension were obtained in the samemanner as in Example 1, except that the cellulose acetate shown in Table1 was used. The results of each measurement are shown in Table 1.

Example 2

A membrane-forming solution was prepared from 20% by weight of thecellulose acetate shown in Table 1, 79% by weight of dimethyl sulfoxideand 1% by weight of lithium chloride in the same manner as in Example 1.A hollow fiber membrane with the same dimension was prepared from theresulting membrane-forming solution in the same manner as in Example 1.The results of each measurement are shown in Table 1.

Comparative Example 4

A hollow fiber membrane with the same dimension was obtained in the samemanner as in Example 2, except that the cellulose acetate shown in Table1 was used. The results of each measurement are shown in Table 1.

Example 3

A membrane-forming solution was prepared from 20% by weight of thecellulose acetate shown in Table 1, 70% by weight ofN-methyl-2-pyrrolidone and 10% by weight of ethylene glycol in the samemanner as in Example 1. The resulting membrane-forming solution wasdischarged from outside of a double pipe type spinning orifice at apressure of 0.6 MPa and at a discharge temperature of 90° C., whilewater of 70° C. as an inside coagulating solution was discharged from aninner pipe. After passing the air for 0.5 sec, it was coagulated in awater bath at 30° C. and spun at a wind-up rate of 12 m/min. Then, thesolvent was removed sufficiently in a washing bath. The resulting hollowfiber membrane had an inner diameter of 0.8 mm and an outer diameter of1.3 mm. The results of each measurement is shown in Table 1.

Comparative Example 5

A hollow fiber membrane with the same dimension was obtained in the samemanner as in Example 3, except that the cellulose acetate shown in Table1 was used. The results of each measurement are shown in Table 1.

TABLE 1 Cellulose acetate Cellulose acetate semipermeable membraneα-cellulose 6% acetylation pure water tensile strength degradationcontent viscosity degree permeation rate at break by a (wt %) (mPa · S)(%) (L/m² · h) (MPa) microorganism Example 1 99.8 96 61.4 510 5.6 notdegradable Comparative 99.8 275  60.8 — — — Example 1 Comparative 97.693 61.6 430 4.7 not degradable Example 2 Comparative 99.7 95 55.6 4905.2 degradable Example 3 Example 2 99.8 68 61.0 580 5.7 not degradableComparative 97.6 93 61.6 550 5.1 not degradable Example 4 Example 3 99.896 61.4 230 5.8 not degradable Comparative 97.6 93 61.6 180 5.6 notdegradable Example 5

As is evident from comparisons between the counterparts, that is,between Example 1 and Comparative Examples 1 to 3, Example 2 andComparative Example 4, and Example 3 and Comparative Example 5,respectively, the cellulose acetates in Examples 1 to 3 satisfied theprescribed 3 elements acting synergistically, resulting in superiorityin any items of permeation rate, tensile strength at break and microbialdegradability. Further, whether pinholes were present or not wasconfirmed by sealing one end of each hollow fiber membrane (dry weight:100 g) in Examples 1 to 3, pressurizing the other end with a nitrogengas at 300 kPa for 10 minutes or more, and then determining the numberleaked, and as a result, no pinhole was recognized in any of themembranes. The viscosity was high in Comparative Example 1, so thatspinning was not feasible even at a discharge pressure of 0.7 MPa at thetime of spinning.

Example 4

A membrane-forming solution consisting of 18 wt % cellulose acetate(degree of acetylation: 61.0%, the viscosity of 6%: 120 mPa·s,α-cellulose content: 99.5 wt %), 81 wt % dimethyl sulfoxide and 1 wt %lithium chloride was discharged from outside of a double pipe nozzle(1.5×0.63×0.33), while water of 70° C. as an inside coagulating solutionwas discharged from an inner pipe. In this step, the amount of themembrane-forming solution discharged was 12 g/min, while the amount ofthe inside coagulating solution discharged was 30 g/min, and the ratioof linear velocity of both of the fluids was 11.2. After passing the airfor 1 second, the membrane-forming solution was coagulated from theouter surface thereof in a coagulation bath at 70° C., and then immersedin a water bath at 50° C. to remove the solvent. The resulting hollowfiber membrane had an inner diameter of 0.8 mm and an outer diameter of1.3 mm, and the spinning rate was 12 m/min. An electron microphotographof a cross-section of the resulting hollow fiber membrane at amagnification of ×50 is shown in FIG. 1. The ratio of the area occupiedby the void part to the total cross-sectional area of the membrane was35%, the pure water permeation rate when the inside of the hollow fibermembrane was pressurized with pure water at 100 kPa was 750 L/m²h, theγ-globulin permeability was 10%, and the tensile strength was 5.8 MPa.FIG. 2 shows an electron microphotograph of the internal surface of theresulting membrane at a magnification of ×20,000. A slit structure of0.1 to 0.3 μm in length and 0.01 to 0.05 μm in width was observed on theinternal surface.

Comparative Example 6

A hollow fiber membrane was prepared in the same manner as in Example 4,except that the membrane-forming solution of Example 4 was dischargedfrom outside of a double pipe nozzle (2.0×0.80×0.50), while water of 60°C. as an inside coagulating solution was discharged from an inner pipe,the amount of the membrane-forming solution discharged was 15 g/min,while the amount of the inside coagulating solution discharged was 22g/min, and the ratio of linear velocity of both the fluids was 5.1. Anelectron microphotograph of a cross-section of the resulting hollowfiber membrane at a magnification of ×50 is shown in FIG. 3. The ratioof the area occupied by the void part to the total cross-sectional areawas 30%, the pure water permeation rate when the inside of the hollowfiber membrane was pressurized with pure water at 100 kPa was 450 L/m²h,the γ-globulin permeability was 10% and the tensile strength was 5.5MPa. FIG. 4 shows an electron microphotograph of the internal surface ofthe resulting Membrane at a magnification of ×20,000. No slit structurewas observed on the internal surface.

Example 5

A membrane-forming solution consisting of 18 wt % cellulose acetate(acetylation degree: 61.0%, 6% viscosity of 6%: 160 mPa·s, α-cellulosecontent: 99.5 wt %), 81 wt % dimethyl sulfoxide and 1 wt % lithiumchloride was discharged from outside of a double pipe nozzle(1.5×0.63×0.33), while water of 70° C. as an inside coagulation solutionwas discharged from an innerpipe. In this step, the amount of themembrane-forming solution discharged was 12 g/min, while the amount ofthe inside coagulating solution discharged was 32 g/min, and the ratioof linear velocity of both of the fluids was 10.1. After passing the airfor 2 seconds, the membrane-forming solution was coagulated from theouter surface thereof in a coagulation bath at 70° C. , and thenimmersed in a water bath at 50° C. to remove the solvent. The resultinghollow fiber membrane had an inner diameter of 0.8 mm and an outerdiameter of 1.3 mm, and the spinning rate was 6 m/min. The ratio of thearea occupied by the void part to the total cross -sectional area of theresultant hollow fiber membrane was 40%, the pure water permeation ratewhen the inside of the hollow fiber membrane was pressurized with purewater at 100 kPa was 1000 L/m²h, the γ-globulin permeability was 20%,and the tensile strength was 5.0 MPa. FIG. 5 shows an electronmicrophotograph of the inner surface of the membrane at a magnificationof ×20,000. A slit structure of 0.1 to 0.3 μm in length and 0.01 to 0.05μm in width was observed on the inner surface.

Comparative Example 7

A hollow fiber membrane was prepared in the same manner as in Example 4,except that a membrane-forming solution consisting of 18 wt % celluloseacetate (acetylation degree: 61.0%, 6% viscosity:160 mPa·s,α-cellulosecontent:99.5wt %) 80 wt % N-methyl-2-pyrrolidone and 2 wt %ethylene glycol was discharged from outside of a double pipe nozzle(2.0×0.80×0.50), while water of 60° C. as an inside coagulation solutionwas discharged from an inner pipe, the amount of the membrane-formingsolution discharged was 14 g/min, while the amount of the insidecoagulating solution discharged was 19 g/min, and the ratio of linearvelocity of both of the fluids was 6.6. The ratio of the area occupiedby the void part to the total sectional area of the resulting hollowfiber membrane was 28%, the pure water permeation flow rate when theinside of the hollow fiber membrane was pressurized with pure water at100 kPa was 350 L/m²h, the γ-globulin permeability was 8%, and thetensile strength was 5.1 MPa. FIG. 6 shows an electron microphotographof the internal surface of the resulting membrane at a magnification of×20,000. No slit structure was observed on the internal surface.

What is claimed is:
 1. A cellulose acetate hollow fiber membrane,wherein the thickness of the hollow fiber membrane is 100 to 400 μm, thecross-section of the hollow fiber membrane is composed of athree-dimensional network part and a void part, the void part ispositioned inside 10 μm or more from both internal and external surfacesof the membrane, the area occupied by the void part is in the range of 5to 60% of the total cross-sectional area of the membrane, a dense layerhaving a surface average pore size of 0.001 to 0.05 μm exists on each ofthe internal and external surfaces of the hollow fiber, and a slitstructure is observed on the internal surface of the hollow fiber withan electron microscope at a magnification of ^(×)20,000.
 2. Thecellulose acetate hollow fiber membrane as claimed in claim 1, whereinthe cellulose acetate is produced from cellulose having an α-cellulosecontent of not less than 99% by weight, and has a 6% viscosity of 50 to200 mPa·s and an acetylation degree of 60.5 to 61.5%.
 3. The celluloseacetate hollow fiber membrane as claimed in claim 2, which has a purewater permeation rate of not less than 500 l/(m^(2·)h) and a tensilestrength at break of not less than 4 MPa.
 4. A process for producing thecellulose acetate hollow fiber membrane as claimed in claim 1 comprisingdissolving a cellulose acetate in a water-soluble, organic polar solventand then discharging the resulting membrane-forming solution from adouble pipe nozzle while discharging an inside coagulating solution fromthe inner pipe of the double pipe to coagulate the membrane-formingsolution in a coagulation bath.
 5. The process for producing a celluloseacetate hollow fiber membrane as claimed in claim 4, wherein the linearvelocity of the inside coagulating solution discharged is 8 times ormore greater than the linear velocity of the membrane-forming solutiondischarged.
 6. The process for producing a cellulose acetate hollowfiber membrane as claimed in claim 4, wherein the membrane-formingsolution contains a compound containing metallic elements of the I toIII groups of the periodic table in an amount of 0.2 to 1.5% by weighttherein.